1. Field of the Invention
The present invention relates to an image processing apparatus, which sends attribute information to a unit for selecting whether to perform printing in only a black coloring material or to perform printing in a plurality of coloring materials based on the attribute information, a method for controlling the image processing apparatus, and a program for implementing the image processing and the control method.
2. Description of the Related Art
Conventionally, for example, when printing is performed, a printing apparatus (printer), which prints color page or monochrome page information, converts page description language (PDL) data into intermediate data, which is called a display list, and rasterizes the intermediate data into RGB bitmap image data. The printing apparatus converts the RGB bitmap image data into CMYK bitmap image data, and then supplies the converted image data to a printer engine.
The PDL data is described by a rendering command for rendering various objects. Commonly known rendering commands include, for example, PS, PCL, LIPS, and PDF. The rendering command is different for each PDL data.
The printing apparatus receives the PDL data, interprets the rendering command by processing called “interpreter”, and converts the PDL data into intermediate data called a display list, in which the rendering commands are listed. When converting the PDL data into the intermediate data, different rendering commands in PS, PCL, LIPS, and PDF, are converted into a common rendering command. Next, the intermediate data is rasterized into RGB bitmap image data by processing known as “rendering”. Simultaneously, attribute information is generated based on a description of the rendering command. The attribute information is information for indicating what kind of region the object to be rendered by the rendering command belongs to. The attributes which may be included in the attribute information include a text attribute, a graphic attribute, and an image attribute. The generated attribute information is assigned to a pixel unit of RGB bitmap image data, and is held in association within association with each pixel of the RGB bitmap image data.
When a pixel value of a target pixel of the RGB bitmap image data is R=G=B, there is a function called “gray compensation” which determines whether CMYK coloring materials are used or whether only a black (K) coloring material is used, based on the attribute information in the target pixel. Technology concerning gray compensation is discussed in Japanese Patent Application Laid-Open No. 2003-320715.
In gray compensation, if the attribute information of the target pixel is a text attribute or a graphic attribute, to prevent the problem of color misregistration caused by mixing of the CMYK coloring materials, only a black coloring material is used. On the other hand, with respect to an image attribute, since there is a limit in the gradation range when printing with only a black coloring material, emphasis is placed on gradation, and CMYK coloring materials are used.
The reason why printing which uses CMYK coloring materials has better gradation than printing which uses only a black coloring material will now be described. First, to perform printing using only a black coloring material, when the pixel value of the RGB bitmap image data is R=G=B, C=M=Y=0%. Therefore, CMYK bitmap image data in which only K has a concentration, has to be generated. This CMYK bitmap image data is termed “K monochromatic bitmap image data”. Here, the gradation range of the K monochromatic bitmap image data is in a K concentration range of 0 to 100% (integer). Namely, for K monochromatic bitmap image data, an intermediate concentration between a concentration of C=M=Y=0% and K=30% and a concentration of C=M=Y=0% and K=31% cannot be expressed. On the other hand, to perform printing using CMYK coloring materials, when the pixel value of the RGB bitmap image data is R=G=B, CMYK bitmap image data having concentrations of C=c %, M=m %, Y=y %, and K=k % has to be generated. The CMY concentrations of c %, m %, and y % do not have to be perfectly the same. For example, C=31%, M=32%, Y=30%, and K=0% is acceptable. More specifically, in the case of CMYK bitmap image data, an intermediate concentration between a concentration of C=30%, M=30%, Y=30%, and K=0% and a concentration of C=31%, M=31%, Y=31%, and K=0% may be expressed, for example, as follows: C=31%, M=30%, Y=30%, K=0%
C=31%, M=31%, Y=30%, K=0%
C=30%, M=30%, Y=30%, K=1%
Thus, a print which uses CMYK coloring materials can have better gradation than printing which uses only a black coloring material.
However, when the printing apparatus receives bitmap image data of a scanned paper document as the PDL data, the PDL data may be compressed in a compression format for compressing a monochrome image such as MMR, JBIG, and PNG. In this data compressed in such a compression format, the attribute information is generated as an image attribute even if the pre-compression object was text, for example. Therefore, when selecting whether to perform printing in only a black coloring material or to perform printing in a plurality of coloring materials based on the attribute information, the printing is performed by a non-optimal coloring material for the object. Consequently, image quality is deteriorated. For example, when the above-described gray compensation is applied, CMYK coloring materials are used even if the rendering color of the object is monochrome text. As a result, the image quality deteriorates due to color misregistration caused by mixing of the CMYK coloring materials.